The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery. Such screws have a head that can swivel about a shank of the bone screw, allowing the head to be positioned in any of a number of angular configurations relative to the shank.
Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate rods are often utilized that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury or disease. Such rods must be supported by certain vertebrae and support other vertebrae.
The most common mechanism for providing vertebral support is to implant bone screws into certain bones which then in turn support the rod or are supported by the rod. Bone screws of this type may have a fixed head relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do. Therefore, polyaxial bone screws are commonly preferred.
Polyaxial bone screws allow rotation of the head about the shank until a desired rotational position of the head is achieved relative to the shank. Thereafter, a rod can be inserted into the head and eventually the head is locked or fixed in a particular position relative to the shank.
A variety of polyaxial or swivel-head bone screw assemblies are available. One type of bone screw assembly includes an open head that allows for placement of a rod within the head. A closure top or plug is then used to capture the rod in the head of the screw.
Because such implants are for placement within the human body, it is desirable for the implant to have as little effect on the body as possible. Consequently, heavy, bulky implants are undesirable and lighter implants with a relatively small profile both in height and width are more desirable. However, a drawback to smaller, lighter implants is that they may be more difficult to rigidly fix to each other and into a desired position. Lack of bulk may also mean lack of strength, resulting in slippage under high loading. Also, more component parts may be required to rigidly fix the implant in a desired position. A further drawback of smaller components is that they may be difficult to handle during surgery because of their small size, failing to provide adequate driving or gripping surfaces for tools used to drive the shank into bone.
One undesirable attribute of some of the swivel-head implants is the need for a multitude of components that may loosen or even disassemble within the body. It is most undesirable for components to be free to move around in the body after the completion of surgery. Loosening of components relative to each other may result in related undesirable movement of the bone or vertebra that the implant was intended to stabilize.